1. Field of the Invention
The present invention relates to a drive unit of a self-luminous device such as an organic electroluminescent device and the like.
2. Description of the Related Art
An image display device used in a portable terminal such as a hand-held mobile phone and the like requires a thin-profile display panel. As the conventional thin-profile display panel, a liquid crystal display panel is generally used. However, a display panel which is constituted of a matrix of a plurality of organic electroluminescent devices, hereinafter called organic EL devices, is more preferable as the image display device for portable terminal, because the display panel with the organic EL devices is not only thin but also lightweight.
Two methods are generally used to drive the organic EL device, those are, a current driving method and a voltage driving method. The organic EL device emits light, luminance of which is corresponding to a supplied current level, so that the drive unit adopting the current driving method keeps a current supplied to the organic EL device at a constant current level, and the drive unit adopting the voltage driving method keeps voltage applied to the organic EL device at a constant voltage level.
However, since the organic EL device is a self-luminous device, a current-luminance characteristic is varied depending on cumulative driving period and the operating environment. When the organic EL device is driven with a constant current, the luminance decreases as the driving time increases. On the other hand, the luminance increases as the ambient temperature increases, and it decreases as the ambient temperature decreases. When the organic EL device is driven with a constant voltage, the rate of variation in the luminance is larger than that in the case where the organic EL device is driven with the constant current. This is because the amount of the current flowing through the organic EL device changes as a consequence of the variation in impedance of the organic EL device depending on the driving time and the operating environment.